The Massachusetts Bay Transportation Authority would spend more than $310 million over the next five years to make the transit system one of the most accessible in country to disabled riders, under a settlement announced yesterday by T officials and groups representing people with disabilities.

The agreement calls for upgrading elevators and escalators, speeding the purchase of low-floor buses, replacing unreliable mobile wheelchair lifts on subway and trolley platforms, accelerating a new $23 million public address system, training MBTA employees with the help of disabled riders, and creating an assistant general manager for accessibility who will answer directly to the T's general manager.

''Certainly for 20 to 30 percent of our customers at the MBTA, we are an essential component in their ability to live their life, enjoy their lives, go to work, go to a movie, and to get there in a reliable fashion," said MBTA General Manager Daniel A. Grabauskas. ''Therefore, it merits a substantial investment."

The settlement still needs the approval of the federal judge overseeing a four-year-old class-action lawsuit against the T under the Americans with Disabilities Act. If it is approved, a court-appointed monitor would oversee progress, which would include using undercover passengers. After the five years covered by the pact, the monitor would have to give passing grades on three quarterly reports before the settlement officially ends.

Daniel S. Manning, lead counsel for the 11 plaintiffs and the Boston Center for Independent Living, said 100 depositions and other information gathered for the lawsuit detailed extensive impediments facing the disabled throughout the T system.

For example, when many elevators and escalators were not working last year, some of the plaintiffs in the lawsuit told of having to take both outbound and inbound trains simply to reach a working elevator or to change tracks.

''One of our goals is to really try and bring riders back" through this settlement, said William Henning, executive director of the Boston Center for Independent Living. ''We know a number of folks who have had problems and haven't used the system in two, three, four years, and it's time we get them back in as part of the mainstream community."

Both Manning and Henning said the settlement talks over the last nine months were the most productive they ever had with the T. They credited the agency with being open to change. ''I know my clients are very excited both to have a seat at the table and to be partners going forward in this," Manning said.

The settlement calls for no monetary damages to be paid to the plaintiffs, though the T will incur all or a portion of the plaintiffs' legal costs. That figure was not disclosed yesterday.

The list of settlement terms includes:

Spending $122 million over the next five years to add, replace, or upgrade elevators and escalators and to ensure continued, uninterrupted service. Park Street, Downtown Crossing, Harvard Square, and Porter Square stations would get additional elevators.

Involving disabled passengers in rider-assistance training for MBTA employees. In past training, T employees either viewed videos or practiced maneuvering wheelchairs with fellow T employees who were not disabled.

Continuing to buy accessible low-floor buses, which are easier for disabled people to board. The T has about 610 of those buses now and plans to get 400 more so that they would be on almost all T routes by 2007.

Closing platform gaps on subway cars and repairing and replacing the tactile yellow warning strips at the edge of some subway platforms.

Whenever possible, assigning one low-floor car to each streetcar train on the Green Line.

T officials and the plaintiffs declined to release a copy of the settlement, which includes the deadlines, until US District Judge Morris E. Lasker signs off on it.

Grabauskas made accessibility his top priority when he took over the agency about a year ago. The settlement now makes those pledges binding, with little or no room for the T to delay accessibility projects.

Much of the funding for these changes was already part of the MBTA's capital budget. T officials could not say yesterday how much more they are spending to reach the settlement's terms.

''Every investment that we're going to make that's essential for a subset of our customers in the disability community is going to be a huge plus for every one of our customers," Grabauskas said.

For instance, getting rid of the cumbersome wheelchair lifts, mostly on the Green Line, should speed service for all riders.

Many passengers also complain about not being able to understand public address announcements, which Grabauskas compared to Charlie Brown's mumbling teacher.

The lawsuit, which was filed in 2002, accused the T of failing to keep elevators and escalators in good repair, preventing the disabled from riding buses and trains 10 percent to 20 percent of the time.

According to the lawsuit, the groups said the T had 1,900 elevator failures in 2004.

The number of broken or out-of-service escalators and elevators at T stations hit a five-year high at the start of 2005, prompting the Federal Transit Administration to begin monitoring the problem.

On some days in the late winter and early spring of last year, more than 20 percent of the MBTA's 167 escalators and more than 15 percent of the 143 elevators were not working, T documents indicate.

Those numbers have vastly improved: 98 percent of the system's elevators have been in full operation in the last two months, official say.